In addition to its interaction with oxygen, hemoglobin in both its ferric and ferrous oxidation states interacts with oxides of carbon, nitrogen and sulfur. We carried out a pilot study in which the interaction of one such ligand, Nitric oxide (NO) with a number of chemically modified hemoglobins using the stopped flow apparatus at Duke University Marine Laboratory, NC. Rapid mixing experiments of (NO) with ferric forms of these hemoglobins show a biphasic kinetics similar to that of unmodified hemoglobin. However, initial analysis showed an appreciable difference in the rate of reactions of modified hemoglobins with (NO). (NO) interactions with modified hemoglobins are of particular interest since the endothelial derived relaxing factor (EDRF) is now believed to be (NO). These experiments provide a framework for in vivo studies on the nature of the interactions of EDRF with various modified hemoglobins that may shed some light on the vasoconstrictive activity of hemoglobin solutions. Recently, a microvolume stopped flow spectrofluorimeter has been installed in our lab. It is a turn key kinetic work station that features absorption, fluorescence and dual wavelength capabilities. A 32 bit RISC processor provides a complete data acquisition and processing package along with a graphics based file screen management system. One can begin to address the question of how changes in oxygen affinity affect ligands binding properties of crosslinked hemoglobins in relation to the overall balance of oxygen loading and unloading in tissue. This may ultimately bear directly on the efficacy and toxicity of the memoglobin preparations.